[ISSUE #B5] Reject query assignment requests with missing required body fields instead of failing with NPE

[zjncs]

Motivation

QUERY_ASSIGNMENT and SET_MESSAGE_REQUEST_MODE requests whose body is missing topic, consumerGroup or messageModel (e.g. sent by an old/foreign client or a hand-crafted request — the JSON body simply leaves the field unset) currently fail deep inside QueryAssignmentProcessor with a NullPointerException instead of a proper error response:

• getMessageRequestMode(null, ...) / findSubscriptionGroupConfig(null) / getConsumerGroupInfo(null) → ConcurrentHashMap.get(null) throws NPE (Object.hashCode() ... because "key" is null);
• doLoadBalance does switch (messageModel) → NPE on the null enum (MessageModel.ordinal() ... is null) — the messageModel field has no default in QueryAssignmentRequestBody;
• setMessageRequestMode calls topic.startsWith(...) on a null topic → NPE.

The client only receives an opaque SYSTEM_ERROR carrying the NPE stack. Guarding the required body fields up front and answering INVALID_PARAMETER matches how other processors (e.g. AdminBrokerProcessor, PollingInfoProcessor) respond to malformed requests.

Modifications

• queryAssignment: reject with INVALID_PARAMETER when topic/consumerGroup is empty or messageModel is null, before any table lookup or doLoadBalance.
• setMessageRequestMode: reject with INVALID_PARAMETER when topic/consumerGroup is empty, before the retry-topic check and the config lookups.

Verification

Fail-before (each of the 5 new tests errors on unpatched code with the NPE described above), e.g.:

testQueryAssignmentWithNullTopic » NullPointer  Cannot invoke "Object.hashCode()" because "key" is null
testQueryAssignmentWithNullMessageModel » NullPointer  Cannot invoke "MessageModel.ordinal()" because "messageModel" is null
testSetMessageRequestModeWithNullTopic » NullPointer  Cannot invoke "String.startsWith(String)" because "topic" is null

Pass-after — full QueryAssignmentProcessorTest (5 existing + 5 new tests) is green:

mvn -pl broker test -Dtest='QueryAssignmentProcessorTest'
Tests run: 10, Failures: 0, Errors: 0, Skipped: 0

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[RockteMQ-AI]

Summary

Adds null checks for topic, consumerGroup, and messageModel in QUERY_ASSIGNMENT and SET_MESSAGE_REQUEST_MODE request handlers — returns a proper error response instead of an NPE when a malformed/incomplete request arrives. Consistent with the existing validation style in the codebase. Thorough test coverage (5 new tests).

LGTM.

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